Phono stage phono loading!?

With the vinyl resurgence, there are many new entrants to the analogue-playback community. Hence, there has been quite a lot of discussion about phono stages, and the discussion inevitably comes around to "phono loading" as most phono-stages come with loading options (either dip switches, plugs, or in some exotic cases remote control!!) many beginners ask how do we determine what loading to use.

The fact is that for low-output moving coil cartridges (the majority of high-end, high-resolution cartridges), loading is often unnecessary and can be detrimental. Here's why:

A phono cartridge is an electrical generator - the inductance of the cartridge plus the capacitance of the tonearm cable results in a LC resonant tank circuit. This circuit has a peak resonance, and this resonance peak can be easily calculated. For low output moving coil cartridges, it is typically in the megahertz range.

Resistance loading helps by knocking this down. However, it is only with extreme values that the loading peak comes down below 1MHz. Some very long, very high-C tonearm cable designs possibly reach the values that might necessitate a bit of additional loading. So loading is useful only with some phono stage designs, and is absolutely critical with phono stages that will overload with high frequency. Many budget models use a cheap opamp that don't have good overload characteristics. Nothing wrong with this, but then loading is extremely beneficial as these resonant peaks can reach 20dBV or more (albeit at frequencies that even bats can't hear).

For the curious, here's an online calculator for an unloaded LC resonant tank circuit (the cartridge plus tonearm cable).

You also have to look at a cartridge as an electrical generator. High loading (we are talking about values below 100 ohms) will strain the ability of the cartridge to deliver current. The result is damping that can be beneficial if there are coil, cantilever or other mechanical resonances that need to be controlled. However, such damping will reduce the dynamic ability of the playback system.

Also, some systems have 1-inch or larger tweeters that beam or tweeter domes/diaphragms that resonate. High loading also can be beneficial as system bandwidth rolls off (again with high loading) - which makes the system sound smoother. In this case, the problems in the phono side help with problems in the loudspeaker.

This is why some tonearm cables sound "wonderful". They have low capacitance and high inductance, and result in the cartridge/phono stage loading system being a RL low-pass filter. At values typically found in our high-end hifi systems, it can make the system sound smooth and detailed by filtering off above 15kHz.

Here's the online calculator if anyone has the time/curiosity:

So, once you know this, it is easy enough to design a system that will require no loading. If loading is needed, there is usually a problem somewhere in the system. As one marketing genius told me a long time ago - "That's not a problem. Spin it another way and it's a feature."

Gary Koh